The detection of surface inclination (i.e., the orientation a surface relative to an XY reference plane) and surface displacement (i.e., the off-set of a surface in the Z-direction relative to an XY reference plane), referred to herein collectively as "surface position," is important in manufacturing of products having a planar surface whose position during the manufacturing process needs to be known with a high degree of precision. Such products include, for example, liquid crystal panels, semiconductor integrated circuits, thin-film magnetic heads, change-coupled devices (CCDs), and the like.
To determine the inclination of a surface in general, it is sufficient to detect the relative displacement of two points on the surface. Position detection apparatuses for measuring the position of a surface by obliquely illuminating the surface with light and detecting the light obliquely reflected therefrom are known. For example, such surface position detection apparatuses are described at Japanese Unexamined Patent Application (Kokai) No. S56[1981]-42205, Japanese Unexamined Patent Application (Kokai) No. H5[1993]-129182, and Japanese Unexamined Patent Application (Kokai) No. H5[1993]-204166.
The surface displacement detection system described at Japanese Unexamined Patent Application (Kokai) No. H5-204166 is, in particular, well-suited for detection of surface displacement of large substrates. This surface displacement detection system is arranged such that it is equipped with two light sources and two detectors. The light from each light source is split by a half prism, thereby forming two light beams, which are guided to two detection locations on the surface being measured. Light from each light beam is reflected from each detection location and is respectively received by the two detectors, which allows for the displacements at the respective detection locations to be measured. The two light sources turn on and off in time division fashion to permit simultaneous measurement of the respective displacements of two points. The displacements of the detection locations at a total of four locations broadly separated from one another around the periphery of a large substrate can thus be detected using two light sources and two detectors. For each such detection location, the inclination of the substrate in the direction of a line drawn between it and another point may be determined from the relative difference in displacements between the two points.
However, there are problems associated with using this type of surface inclination detection apparatus. For instance, because the detection locations are detected in a time-division fashion, detecting of all of the many detection locations on the detection target surface and determining the inclination of the substrate has been time consuming, which reduces the ability to process substrates in a given time interval (i.e., reduces "throughput"). Furthermore, attempts to remedy this problem with similar arrangements requires respective optical systems for the several detection locations, resulting in a complicated apparatus. This represents a serious impediment from the standpoints of design and manufacturing, and has resulted in excessive expenditure of labor.